1. Field of the Invention
This invention relates to medical instrumentation for achieving spinal fusion and, more particularly, to a novel, highly adaptive, and interchangeable component system and method for fixation of the lumbar spine and lumbosacral spine to aid the fusion of these spinal regions.
2. The Prior Art
The spine is a flexible, multi-segmented column that supports the upright posture in a human while providing mobility to the axial skeleton. The lumbar spine serves the functions of encasing and protecting vital neural elements and provides structural support for the body by transmitting the weight of the body through the pelvis to the lower extremities. Since there are no ribs attached to the lumbar spine, it has a relatively wide range of motion.
The spine is made up of bone, intervertebral discs, synovial joints with their articular cartilage, synovial capsules and, as part of the back, is surrounded by supporting ligaments, muscle, fascia, blood vessels, nerves, and skin. As in other areas of the body, these elements are subject to a variety of pathological disturbances: inflammation, trauma, neoplasm, congenital anomalies, disease, etc. In fulfilling its role in the back, the spine can be subjected to significant trauma which is assumed to play a dominant role in the etiology of low back pain. Trauma frequently results in damage at the upper end of the lumbar spine, where the mobile lumbar segments join the less mobile dorsal spine. Excessive forces on the spine can not only produce life-threatening traumatic injuries, but may contribute to an increased rate of degenerative change. Degenerative changes tend to develop in the lower lumbar intervertebral discs, most commonly in the third decade. Osteoarthritis produces changes in the facet joints by middle age.
Certain severe cases of spine anomalies, such as a congenital scoliotic spine, or scoliosis developed as the result of diseases as cerebral palsy or muscular dystrophy, require surgery and instrumentation to correct or at least lessen the anomalous spine curvature. If exterior brace treatment has failed or a large progressive curve has developed without treatment, surgery can be used to diminish the curve. Severe scoliosis that goes untreated can cause deformities of the ribs that restrict the lung and later cause serious breathing problems, heart disease, or severe back pain.
One of the methods used to treat disabling pain, neurological compromise, or deformity produced by any of the above noted pathological conditions is fusion. The earliest spinal fusion techniques were basically posterior interlaminar fusions. Subsequently, later techniques led to the evolution of posterolateral techniques that allow a larger area for bone grafting and fusion. The relatively high rate of unsatisfactory results with traditional fusion techniques led to the evolution of lumbosacral fusion involving the use of hardware or instrumentation in an attempt to achieve stability and thus fusion or correction of deformity and stability followed by fusion. Internal spinal fixation increases rigidity and results in a high rate of fusion. This increased fusion rate and decreased pseudarthrosis rate gives better results and can significantly reduce postoperative pain and time for convalescence. Spinal fixation using instrumentation also allows correction of deformities and maintenance of that correction during consolidation by fusion. The primary considerations of the indications for spinal instrumentation are the magnitude of instability, the plane of deformity, and the available intact anatomy.
The past decade or two has seen an extensive development of internal devices for the lumbar and lumbosacral spine fixation. The following patents are typical of the patents in this field: Edwards (U.S. Pat. No. 4,569,338) teaches a sacral fixation screw having an aperture in the top for the engagement of a hook. Steffee (U.S. Pat. No. 4,648,388) teaches an apparatus for imposing a force on the spinal column. Howland et al. (U.S. Pat. No. 4,653,481) teach a spine fixation system having a plurality of screw clamp assemblies inserted through the pedicle and vertebral body and affixed to a rigid rod. Steffee (U.S. Pat. No. 4,719,905) teaches an apparatus including a rod, a plurality of clamps, and a plurality of fastener assemblies for securing the rod to a spinal column. Puno et al. (U.S. Pat. No. 4,805,602) teach an apparatus for the internal fixation of the spine, the apparatus including two sets of implants each consisting of a rod and a plurality of vertebral anchors. Heinig et al. (U.S. Pat. No. 4,887,595) teach a plate and screw system for maintaining the relative position of the spinal bodies of a spinal column. Sherman (U.S. Pat. No. 4,887,596) teaches a pedicle screw for use in internal fixation of the spine. Gotzen et al. (U.S. Pat. No. 4,944,743) teach an implantable fixation device having a support bar with jaw supports threadedly engaged to the support bar. Gaines, Jr. (U.S. Pat. No. 4,950,269) teaches a rod and fastener apparatus for connecting the rod to the vertebra of a spinal column. Krag et al. (U.S. Pat. No. 4,987,892) teach a pedicle screw and rod apparatus for spinal fixation. Cotrel (U.S. Pat. No. 5,005,562) teaches an implant for spinal fixation, the implant including a rod and pedicle screws and hooks mountable to the rod. Howland (U.S. Pat. No. 5,030,220) teaches an implantable spinal fixation system that uses a pedicle screw to secure the longitudinal rods to the spine. An improved locking system maintains the structural integrity of the construct. Cozad et al. (U.S. Pat. No. 5,074,864) teaches a mid-line clamp assembly for use in posterior spinal fixation. The clamp assembly includes inferior and superior clamp halves that are slideably interconnected. The clamp halves are engageable about the longitudinal rods. Asher et al. (U.S. Pat. No. 5,084,049) teach a pair of corrective devices for securement to a spinal column. Each corrective device includes a spine plate having a plurality of openings for receiving a fastener to connect the spine plate to a vertebra. Tsou (U.S. Pat. No. 5,122,131) teaches an orthopedic device for secure mechanical coupling to an elongated surgical rod. Dubousset (U.S. Pat. No. 5,147,360) teaches an osteosynthesis device for correction of spinal curvature wherein anterior and posterior rods are affixed to the vertebral bodies to apply the necessary corrective forces to the spinal column. Cotrel (U.S. Patent No. 5,154,719) teaches an implant for osteosynthesis, the implant being in the form of a screw having a rod-receiving head. Mathews (U.S. Pat. No. 5,171,279) teaches a percutaneous fusion technique using suprafascial internal fixation. Schlapfer (U.S. Patent No. 5,190,543) teaches a pedicle screw having a slotted head for receiving a support rod. Mehdian (U.S. Pat. No. 5,217,497) teaches an implant for fixing one segment of a spinal column to another segment, the implant being in the form of a screw having a slotted head to which a support rod is anchored. Krag et al. (U.S. Pat. No. 5,219,349) teaches a device for use in the controlled alignment of a fractured spine in conjunction with the Vermont Spinal Fixator. Ashman (U.S. Pat. No. 5,242,445) taches an eyebolt having two shell-like portions for engagement to a spinal rod. Vignaud et al. (U.S. Pat. No. 5,261,907) teach an interconnecting device able to lock two spinal osteosynthesis fasteners. Wagner (U.S. Pat. No. 5,334,203) teaches a medical construct using surgical rods and connectors. The connector includes a plate with a pair of double hook bolts to secure the plate to the surgical rods. Yuan et al. (U. S. Pat. No. 5,352,225) teach a dual-tier spinal clamp locking and retrieving system. Jeanson et al. (U.S. Pat. No. 5,360,429) teach a device for straightening, fixing, compressing, and elongating the cervical spine. Lahille et al. (U.S. Pat. No. 5,380,325) teaches a consolidated rod and plural members such as pedicular screws and vertebral claws. Acromed Corp. (European Patent Application Publication Number 0 553 424 Al) teaches a plurality of screw-like fasteners mounted to individual vertebrae and interconnected by a longitudinal rod.
The most common rationale for using such devices is to reduce the incidence of pseudarthrosis after bone grafting. Another rationale (typically for trauma management) is to maintain intervertebral alignment to protect the neural elements until healing occurs. Yet another rationale is to provide fixation for correction of severe anomalous spine curvature due to severe scoliosis or other deformity which threatens life or health.
One of the early fixation methods involved the placement of screws obliquely across each facet joint involved in the grafting. However, the pseudarthrosis rate for this procedure was unacceptably high. Numerous other types of devices that variously include plates, wires, rods, bolts, hooks, and screws have evolved since that time and have resulted in a plethora of devices and instrumentation apparatus for use by the orthopedic surgeon to accomplish spinal fixation. Some of these fixation apparatus and methods require multiple adjustments to the longitudinal rods in order to adapt to specific anatomy. Although not provided by all these devices, the ideal spinal fixation apparatus would provide internal alignment and fixation not just in any one of various planes of movement, but in a full, three-dimensional construct where subject fixation apparatus is simplified, low profile, and easily manipulated and adjusted by the surgeon to allow for anatomy variations, while at the same time providing an extremely rigid construct upon tightening of connection points.
As shown herein before, numerous patents have been issued for various types of spine fixation devices. These devices employ different mechanical apparatus for enabling the surgeon to selectively adjust the alignment of the patient's spine and then to secure that alignment with the spine fixation device. Most of these devices are relatively difficult to adjust and require undue surgical time in their implantation. Further, due to the wide variation in spinal dimensions and availability of suitable attachment sites, most devices have limited application. Further still, these devices do not allow the surgeon to easily manipulate and position the bone screws prior to final tightening of the device.
Another characteristic inherent in prior art spinal fixation apparatus are the manipulation restrictions due to screw thread configuration. Generally, prior bone screw thread configurations have been either single thread pitch (number of threads per unit of length) over the entire length of the bone screw, which requires that the clamping device be engaged from the beginning of the threading process, or other complex configurations involving set screws, or other devices which are either cumbersome to adjust and tighten or cause undue disruption of the cancellous bone tissue. Bone screws of differing thread pitch have an advantage that final positioning is not required until just prior to final tightening of the device, but still have the disadvantage of, due to the discrepancy in the thread pitch, causing the bone screw to create undue coaxial pressure on the lattice-like cancellous tissue of the bone, thereby increasing possible shearing or undue disruption of bone tissue.
A further characteristic of prior bone screw configurations is that the thread angle of the top surface of the thread is not orthogonal relative to the axis of the screw, thereby lessening gripping strength. Specifically, increasing the angle of the thread surface proximal to the bone screw head such that the angle is nearly orthogonal relative to the bone screw axis, correspondingly decreases the lateral forces imposed by that thread on bone tissue by the tightening process.
In view of the foregoing, it would be a significant advancement in the art to provide a spinal fixation apparatus and method that was highly interchangeable, simplified, and would increase the ease of installation and adjustment while decreasing the total time required for surgical implantation and fixation. It would also be an advancement in the art to provide a spinal fixation apparatus and method that would utilize a multi-diameter threaded bone screw of the same thread pitch for all diameters, such that, upon tightening, would have no net increase in axial pressure on the cancellous bone tissue due to thread pitch variance. An even further advancement in the art would be to provide a bone screw with a modified thread configuration such that the top surface of the thread is nearly orthogonal to the axis of the bone screw, thereby decreasing the lateral pressure on the cancellous bone tissue, subsequently lessening the likelihood of shearing upon tightening. Such a novel spinal fixation apparatus and method is disclosed and claimed herein.